Center of gravity

[member2006]

Hello folks. Can anyone give a good explanation as to why producing thrust for lift above a crafts center of gravity or center mass creates better hover stability than at or below center mass?

[capncrunch]

Well, lets experiment shall we?

take an empty soda can, pull the ring tab up so its vertical. stick your finger in it and lift the can. easy, huh?

now take the can and balance it with the bottom of the can on the tip of your finger. not as easy, eh?

in the first example, you are exerting the force that is holding the can up above the can's CofG. in the second trial, the vertical force that keeps the can from falling is below the CofG.

-b

[member2006]

Thanks capn. Anybody else wanna try?

[ShadowGibbon]

With the Center of Gravity being a point in a 3-dimensional space (insert model here), if you hang the C of G below whatever force is holding it up, all it has to do is hang.
If you put the thrust below the C of G, then you're having to ballance the model on a point, and is inharently unstable.

I must say that Capncrunch's explanation is the exact same as what I just said.

Putting the lifting force at the LEVEL of the C of G, will give a more symetrical performance, however you must also have more than just one lifting force (actually you need 3). Otherwise you have a single lifting force centered directly in the C of G, and if there is any outside force that acts apon it (such as wind), then it will become terminally unstable in no time at all.

[member2006]

I'm starting to see the light. Thanks SG.

[capncrunch]

here we go!
from
http://eaa.aviationuniversity.com/multimedia/img/Stability.gif
(I wonder if they know these images are public... the classes are pass-protected...)

there are 3 types of (static) stability. static in this situation means at a frozen point in time: the plane is hovering, its statically stable. in the top graphic of the picture, "positive static stability", if the ball is caused to roll to either side, it will roll back down to the center. something with positive static stability is said to be stable, its a quality brought about by things like dihedral on trainer wings - if you start to roll a trainer off to the right, it will tend to roll back to neutral.

the second graphic is neutral static stability - if you push the ball to the side, it stays there. this is the ideal for aerobatic planes. if you roll the plane over on its side, it stays exactly where you put it with no bad habits.

the third one is negative stability. the ball stays put on its own, balanced at the top of the hill. given a push to either direction, it falls off and wont return back to the top unless you push it back up juuuuuust right. this is the thrust-below-CG position and is the hardest to maintain hover from; its like balancing a broomstick on your nose.

-barrett

[Salto]

Hello folks. Can anyone give a good explanation as to why producing thrust for lift above a crafts center of gravity or center mass creates better hover stability than at or below center mass?If I understand you're question correctly, you're asking if a tractor configuration is more stable than a pusher? My understanding is that the opposite is true.

Here are some links with discussion on the topic.
http://www.rcgroups.com/forums/showthread.php?t=342037

http://www.rcgroups.com/forums/showthread.php?t=270154&page=2&pp=15

Graham.

[capncrunch]

If I understand you're question correctly, you're asking if a tractor configuration is more stable than a pusher?
Graham.


Graham raises a good point. In terms of a 3d plane, neither configuration has positive static stability overall. Tilt the thrustline far enough away from vertical and both orientations would be unstable. I think its all relative to the size of the rotor disc, and the distance between the center of thrust and the center of mass.

Graham, in one of those threads you talk about the fuselage "lagging behind" the sideways motion of a plane falling out of hover, and also about precessional effects on prop thrust. If the plane rotates slightly out of vertical, there will be a horizontal component of force acting at the prop. As the plane starts moving horizontally, there will be a moment at the center of pressure of the plane related to the speed its moving. And since the CG is now out of line with the vertical component of the thrust, there will be a moment at the CG related to the angle of the thrustline and the vertical component of thrust.

so the moment acting to push the plane farther away from vertical is related the speed its moving laterally and the moment acting to swing the fuse back down to vertical is based on the angle.

precession arises from different angles of attack of the prop blades on different sides of the prop arc. this only happens when the prop disc is moving through the air at an angle to the thrustline. to get a noticeable effect, it has to be moving at a speed that would be well beyond the limit of a stable hover.

This makes sense. In a hover, you can drop away from vertical small amounts, but you need to correct back to vertical before it starts moving or else things get out of hand. In a harrier, you're using the elevator to move the center of pressure so that all the forces balance. Slow the plane down and it will swing back to vertical.

-barrett

[Salto]

.... precession arises from different angles of attack of the prop blades on different sides of the prop arc. this only happens when the prop disc is moving through the air at an angle to the thrustline. to get a noticeable effect, it has to be moving at a speed that would be well beyond the limit of a stable hover....I think you're confusing precession and what's commonly referred to as the "P effect".

Precession in a gyroscopic effect whereby a torque applied to a spinning flywheel (the prop in this case) produces a reaction torque about an axis which is 90 Deg from the axis of the applied torque. It occurs as long as there is an applied torque, whether the prop is moving sideways or not.

The P effect is an offset to the thrust line of a prop when that prop encounters any non-axial flow component. It's caused by the blades seeing different angles of attack as they rotate around the motor shaft axis. P effect manifests itself for example, when a tail dragger plane rolls out for takeoff and the prop disc is not at 90 Deg to the direction of travel.

P effect only occurs when there is a non-axial component of flow. On a hovering model, it must be translating sideways (relative to the air mass it's flying in) for this to happen.

Graham.

[Salto]

.... In terms of a 3d plane, neither configuration has positive static stability overall .....I'm not sure that this is correct. Some people claim a pusher can actually be made stable in a hover. If I remember correctly, the Hiller flying platform referenced in one of the links I gave above, claims to be stable and will fly hands off. It's a pusher.

Graham.

[capncrunch]

I think you're confusing precession and what's commonly referred to as the "P effect".
Graham.

yes this occured to me about 12 hours after I posted :) I think you confused me by putting them both in the same paragraph.

but precession is going to be turning the plane out of vertical in any rotation/configuration. if the plane is rotating in a pitch direction, the gyroscopic force will act in the yaw direction.

If the rotating body is symmetrical and its motion unconstrained, and if the torque on the spin axis is at right angles to that axis, the axis of precession will be perpendicular to both the spin axis and torque axis." http://en.wikipedia.org/wiki/Gyroscopic_precession

this is something we deal with already - it's why right thrust is added to motors (that turn CCW).

as to the hiller, I was trying to steer the argument towards 3d planes.

-barrett